1. Field of the Invention
The present invention relates to a system for controlling the air-fuel ratio of an internal combustion engine.
2. Description of the Prior Art
It is desirable from the standpoint of environmental protection that systems for purifying an exhaust gas emitted from internal combustion engines on automobiles, for example, with a catalytic converter and discharging a purified exhaust gas control the air-fuel ratio of an exhaust gas emitted from an internal combustion engine at an appropriate air-fuel ratio which allows the catalytic converter to have a better ability to purify an exhaust gas.
One conventional air-fuel ratio control system combined with an internal combustion engine has been disclosed in Japanese laid-open patent publication No. 5-321721 which corresponds to U.S. Pat. No. 5,426,935.
The disclosed air-fuel ratio control system has an exhaust gas sensor (air-fuel ratio sensor) disposed in the exhaust system of the internal combustion engine for detecting the air-fuel ratio of an exhaust gas upstream of a catalytic converter, and another exhaust gas sensor (oxygen concentration sensor) disposed in the exhaust system for detecting the concentration of a certain component of the exhaust gas that has passed through the catalytic converter, e.g., the concentration of oxygen (which is commensurate with the air-fuel ratio of the exhaust gas that has passed through the catalytic converter), downstream of the catalytic converter. A basic air-fuel ratio for an air-fuel mixture upstream of the catalytic converter is established depending on the intake pressure and rotational speed of the internal combustion engine. The basic air-fuel ratio is corrected by a PID (proportional plus integral plus derivative) control process such that the oxygen concentration (air-fuel ratio) detected by the exhaust gas sensor down-stream of the catalytic converter will be of an appropriate value, thereby determining a target air-fuel ratio upstream of the catalytic converter. The rate of fuel supplied to the internal combustion engine is feedback-controlled according to the PID control process or an adaptive control process so as to cause the air-fuel ratio detected by the exhaust gas sensor upstream of the catalytic converter to converge toward the determined target air-fuel ratio. In this manner, the air-fuel ratio of the exhaust gas upstream of the catalytic converter is controlled within an appropriate range (window) which enables the catalytic converter to have a good purifying ability, thereby increasing the purifying ability of the catalytic converter.
As a result of various studies made by the inventors, it has been found that in order to keep the catalytic converter effective to purify the exhaust gas regardless of aging thereof, it is necessary to adjust the concentration of a certain component, e.g., the concentration of oxygen, of the exhaust gas downstream of the catalytic converter to a predetermined adequate value with high accuracy.
In the above conventional air-fuel ratio control system, however, since the basic air-fuel ratio upstream of the catalytic converter is corrected by the PID control process to determine a target air-fuel ratio in order to equalize the oxygen concentration detected by the exhaust gas sensor downstream of the catalytic converter to an appropriate value, it is difficult to adjust the oxygen concentration (air-fuel ratio) of the exhaust gas downstream of the catalytic converter highly accurately to an adequate value because of disturbances acting on the exhaust gas sensors and a dead time present in the exhaust system of the internal combustion engine. Particularly, the dead time present in the exhaust system including the catalytic converter has been a large obstacle to the above control process because the dead time is generally long.
Therefore, there has been a demand for a control process capable of eliminating effects of the dead time which is present in the exhaust system including the catalytic converter.
The inventors have attempted to determine a correction quantity for correcting the air-fuel ratio of the internal combustion engine in a sliding mode control process according to the modern control technology for eliminating effects of disturbances, etc.
The sliding mode control process is a feedback control process of variable structure. According to the sliding mode control process, a hyperplane (see FIG. 7 of the accompanying drawings) expressed by a linear function which has as its variables a plurality of state quantities of an object to be controlled is defined in advance, and the state quantities are converged onto the hyperplane under high-gain control. Furthermore, while the state quantities are being converged onto the hyperplane, the state quantities are converged toward a given balanced point (a point of convergence) by an equivalent control input.
The sliding mode control process has excellent characteristics in that once the state quantities are converged onto the hyperplane, the state quantities can stably be converged toward the balanced point without being substantially subject to effects of disturbances, etc.
For determining a correction quantity for the air-fuel ratio of the internal combustion engine in order to adjust the concentration of a component, such as oxygen concentration, of the exhaust gas downstream of the catalytic converter to a predetermined appropriate value, it is possible to regard the value of the concentration of the component of the exhaust gas downstream of the catalytic converter and a rate of change thereof as state qualities of the exhaust system which is an object to be controlled, and determine the correction quantity for the air-fuel ratio in order to converge those state qualities toward a balanced point on a hyperplane, where the value of the concentration and its rate of change are a predetermined appropriate value and "0", respectively, according to the sliding mode control process. If the correction quantity for the air-fuel ratio is determined according to the sliding mode control process, then the concentration of the component of the exhaust gas downstream of the catalytic converter can be adjusted to the predetermined appropriate value with higher accuracy than according to the conventional PID control process or the like.
Various studies made by the inventors, however, have indicated that the general sliding mode control process is susceptible to the dead time present in the object to be controlled and the disturbances until the state quantities of the object to be controlled converge onto the hyperplane.
In the application of the sliding mode control process to the control of the air-fuel ratio of the internal combustion engine, the exhaust system including the catalytic converter suffers a long dead time and various disturbances, as described above.
Therefore, even if a correction quantity for the basic air-fuel ratio is determined according to the sliding mode control process, the state quantities (the value of the air-fuel ratio ad its rate of change) of the air-fuel ratio downstream of the catalytic converter cannot stably be converged onto the hyperplane, with the result that the air-fuel ratio downstream of the catalytic converter cannot stably be converged toward a predetermined appropriate value.